The problem of controlling static charge is well known in the field of photography. The accumulation of charge on film or paper surfaces leads to the attraction of dirt which can produce physical defects. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) can produce irregular fog patterns or "static marks" in the emulsion. Static problems have been aggravated by the increase in the sensitivity of new emulsions, the increase in coating machine speeds, and the increase in post-coating drying efficiency. The charge generated during the coating process may accumulate during winding and unwinding operations, during transport through the coating machines and during finishing operations such as slitting and spooling. Static charge can also be generated during the use of the finished photographic film product. In an automatic camera, the winding of roll film in an out of the film cartridge, especially in a low humidity environment, can result in static charging. Similarly, high speed automated film processing can result in static charge generation. Sheet films (e.g., x-ray films) are especially susceptible to static charging during removal from light-tight packaging.
It is generally known that electrostatic charge can be dissipated effectively by incorporating one or more electrically-conductive "antistatic" layers into the film structure. Antistatic layers can be applied to one or to both sides of the film base as subbing layers either beneath or on the side opposite to the light-sensitive silver halide emulsion layers. An antistatic layer can alternatively be applied as an outer coated layer either over the emulsion layers or on the side of the film base opposite to the emulsion layers or both. For some applications, the antistatic agent can be incorporated into the emulsion layers. Alternatively, the antistatic agent can be directly incorporated into the film base itself.
A wide variety of electrically-conductive materials can be incorporated into antistatic layers to produce a range of conductivities. These can be divided into two broad groups: (i) ionic conductors and (ii) electronic conductors. In ionic conductors charge is transferred by the bulk diffusion of charged species through an electrolyte. Here the resistivity of the antistatic layer is dependent on temperature and humidity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, ionic conductive polymers, polymeric electrolytes containing alkali metal salts, and colloidal metal oxide sols (stabilized by metal salts), described previously in patent literature, fall in this category. However, many of the inorganic salts, polymeric electrolytes, and low molecular weight surfactants used are water-soluble and are leached out of the antistatic layers during processing, resulting in a loss of antistatic function. The conductivity of antistatic layers employing an electronic conductor depends on electronic mobility rather than ionic mobility and is independent of humidity. Antistatic layers which contain conjugated polymers, semiconductive metal halide salts, semiconductive metal oxide particles, etc., have been described previously. However, these antistatic layers typically contain a high volume percentage of electronically conducting materials which are often expensive and impart unfavorable physical characteristics, such as color or reduced transparency, increased brittleness and poor adhesion, to the antistatic layer.
Colloidal metal oxide sols which exhibit ionic conductivity when included in antistatic layers are often used in imaging elements. Typically, alkali metal salts or anionic surfactants are used to stabilize these sols. A thin antistatic layer consisting of a gelled network of colloidal metal oxide particles (e.g., silica, antimony pentoxide, alumina, titania, stannic oxide, zirconia) with an optional polymeric binder to improve adhesion to both the support and overlying emulsion layers has been disclosed in EP 250,154. An optional ambifunctional silane or titanate coupling agent can be added to the gelled network to improve adhesion to overlying emulsion layers (e.g., EP 301,827; U.S. Pat. No. 5,204,219) along with an optional alkali metal orthosilicate to minimize loss of conductivity by the gelled network when it is overcoated with gelatin-containing layers (U.S. Pat. No. 5,236,818). Also, it has been pointed out that coatings containing colloidal metal oxides (e.g., antimony pentoxide, alumina, tin oxide, indium oxide) and colloidal silica with an organopolysiloxane binder afford enhanced abrasion resistance as well as provide antistatic function (U.S. Pat. Nos. 4,442,168 and 4,571,365).
The requirements for antistatic layers in silver halide photographic films are especially demanding because of the stringent optical requirements. Other types of imaging elements such as photographic papers and thermal imaging elements also frequently require the use of an antistatic layer. However, the requirements there maybe somewhat different. For example, for photographic paper, an additional criterion is the ability of the antistatic backing layer to receive printing (e.g., bar codes or other indicia containing useful information) typically administered by dot matrix or inkjet printers and to retain these prints or markings as the paper undergoes processing (viz, backmark retention). Yet another important criterion for photographic paper is its spliceability. Heat splicing of photographic paper rolls is often carried out during printing operations and is expected to provide enough mechanical strength to resist peeling as the web goes through automatic photographic processing. Heat splicing is typically carried out between the silver halide side of the paper and the antistatic backside of the paper. Poor splice strength can cause a number of problems including jamming of automatic processing equipment.
Electrically-conductive layers are also commonly used in imaging elements for purposes other than providing static protection. Thus, for example, in electrostatographic imaging it is well known to utilize imaging elements comprising a support, an electrically-conductive layer that serves as an electrode, and a photoconductive layer that serves as the image-forming layer. Electrically-conductive agents utilized as antistatic agents in photographic silver halide imaging elements are often also useful in the electrode layer of electrostatographic imaging elements.
As indicated above, the prior art on electrically-conductive layers in imaging elements is extensive and a very wide variety of different materials have been proposed for use as the electrically-conductive agent. There is still, however, a critical need in the art for improved electrically-conductive layers which are useful in a wide variety of imaging elements, which can be manufactured at reasonable cost, which are environmentally benign, which are durable and abrasion-resistant, which are effective at low coverage, which are adaptable to use with transparent imaging elements, which do not exhibit adverse sensitometric or photographic effects, and which maintain electrical conductivity even after coming in contact with processing solutions (since it has been observed in industry that loss of electrical conductivity after processing may increase dirt attraction to processed films which, when printed, may cause undesirable defects on the prints).
In addition to controlling static charging, auxiliary layers applied to photographic elements also provide many other functions. These include providing resistance to abrasion, curl, solvent attack, halation and providing reduced friction for transport. One additional feature that an auxiliary layer must provide when the layer serves as the outermost layer is resistance to the deposition of material onto the element upon photographic processing. Such material can impact the physical performance of the element in a variety of ways. For example, large deposits of material on a photographic film lead to readily visible defects on photographic prints or are visible upon display of motion picture film. Alternatively, post-processing debris can influence the ability of a processed film to be overcoated with an ultraviolet curable abrasion resistant layer, as is done in professional photographic processing laboratories employing materials such as PhotoGard, 3M. Finally, processing residue on photographic elements can impact the ability to read magnetically recorded information on a processed film, such as the new advanced photographic system films.
The final step for many photographic processing sequences is the stabilization or final rinse bath. While the components of this final solution vary depending upon the type of photographic material being processed, it is common practice to include one or more surfactants in the final solution. The surfactants are included to improve the wetting of the photographic media being processed. Such wetting should reduce the tendency to form spots upon drying of the media.
While the surfactants in the final processing solution can assist with the wetting of the media, an objectionable surface haze or scum may still appear on the media after processing. Prior art has shown that the outer surface of the media must consist of blends of hydrophobic and hydrophilic materials. U.S. Pat. No. 4,735,976 teaches the use of blends of cellulose nitrate and an interpolymer containing a hydrophobic monomer and an acid monomer to prevent scumming by surfactants in the processing solutions. To prevent spotted drying unevenness after processing, U.S. Pat. No. 4,582,784 teaches the use of blends of cellulosic esters with hydrophilic interpolymers or homopolymers. E.P. 0 644 454 A1 teaches the use of a two layer system that avoids the occurrence of water spot defects after processing. The outermost layer contains a polyoxyalkylene compound as an antistatic agent, while the underlying antistress layer contains a hydrophilic colloid and at least one synthetic clay.
One type of processing scum that is particularly troublesome is hard-water scum. Processing laboratories that are located in hard-water areas are particularly susceptible to this problem. After processing in solutions prepared using hard-water, a white hazy surface scum, sometimes uniform and sometimes more liney and streaky, can be seen on the film. Chemical analysis of the hard-water scum typically reveals hard-water salts of calcium, magnesium, and sodium. As mentioned previously, such a hard-water scum can lead to printable defects or interfere with subsequent coating or magnetic reading operations.
It is toward the objective of providing improved electrically-conductive layers that more effectively meet the diverse needs of imaging elements--especially of silver halide photographic films but also of a wide range of other imaging elements--than those of the prior art that the present invention is directed.
The antistatic layer of the present invention comprises an electrically conducting smectite clay as component A and a hydrophobic, film forming binder comprising an interpolymer of vinylidene halide as component B.
Use of smectite clay in imaging elements has been disclosed before. For example, in European Patent Application 0644454A1 use of synthetic clay in an antistress layer for x-ray films has been disclosed. In U.S. Pat. No. 5,478,709, the use of synthetic clay in the silver halide emulsion layer for reduction in roller marks during automatic processing has been described. The use of synthetic hectorite as an additive to a silica containing antistatic layer has been proposed before in U.S. Pat. No. 4,173,480 and U.S. Pat. No. 5,494,738. However, the integrity of these layers, when present as external layers, in contact with processing solutions during high speed processing is likely to be minimal resulting in loss of post-processing antistatic characteristic. In fact, some of the drawbacks of external antistatic layers containing a combination of hectorite clay and silica applied on photographic paper have been described in commonly assigned copending application U.S. Ser. No. 08/937,685 filed Sep. 29, 1997. Although some binders have been mentioned in U.S. Pat. No. 4,173,480 for their use in conjunction with hectorite clay for application as a surface sizing agent for the fibrous paper base, the binders mentioned therein are hydrophilic binders such as gelatin, starch and methyl cellulose which are likely to offer little resistance to the processing solutions. Again, some of the drawbacks of external antistatic layers containing a combination of hectorite clay and these suggested binders applied on photographic paper have been described in commonly assigned copending application U.S. Ser. No. 08/937,685. The use of an organic compound which can intercalate inside and/or exfoliate smectite clay has been taught in commonly assigned copending applications U.S. Ser. No. 08/937,685 and 08/940,860 for application in antistatic layers containing smectite clay. However, as demonstrated later, antistatic layers, as per the teachings of these patent applications, do not retain sufficient post-process conductivity. Thus the prior art for antistatic layers comprising clay, suitable for imaging elements, is quite inadequate.
As will be demonstrated later, the present invention provides an antistatic layer which is a simple two component system comprising a smectite clay and a hydrophobic, film forming binder which is an interpolymer of vinylidene halide wherein the said antistatic layer unexpectedly provides certain advantages over the teachings of the prior art including the retention of antistatic properties after photographic processing and the prevention of hard-water scum formation after photographic processing.